The proposed research will include (1) an investigation of the mechanism of ATP inhibition of phosphofructokinase (PFK) and (2) a study of the differences that exist in the regulatory as well as structural properties of PFK in tissues of diverse metabolic function. The former will involve a continuation of kinetic analysis of ATP inhibition of hybrids of native enzyme and of enzyme desensitized to allosteric inhibition. The second aspect of the work will involve a survey of rabbit tissues for the presence of PFK isozymes by electrophoretic and immunochemical analyses. Particular focus will be placed on the regulatory properties of a new isozyme found in the brain, the C isozyme. The nature of the phosphofructokinase in platelets and lymphoid tissues will also be examined.